Method of gapping spark plugs



June 5, 1945. R. K. CHRISTIE 2,377,481-

METHOD OF GAPPING SPARK PLUGS Filed March 23, 1943 IN V EN TOR.

F1 1 5 EOBEETK CHE/577E.

Patented June 5, 1945 UNITED STATES PATENT OFFICE 2,377,481 METHOD OFGAPPING SPARK PLUGS Robert K. Christie, Toledo, Ohio Application March 23, 1943, Serial No. 480,175

6 Claims.

This invention relates to spark plugs, and more particularly to methods of gapping electrodes of spark plugs to establish a predetermined spark gap between a multi-prong ground electrode and the center electrode .of aspark plug.

In the manufacture of spark plugs, particularly spark plugs suitable for .use in airplanes, multiprong ground electrodes have usually been provided to give longer gap life under the exacting conditions in which these plugs are. used. The spark gaps of these spark plugs are formed between several prongs of the ground electrode and usually a cylindrical center electrode, In the manufacture of these plugs a problem arises wherein the establishment of a suitable gap between the several prongs of the multi-prong ground electrode and the cylindrical center wire inasmuch as these gaps between the various parts must fall within a very small tolerance, usually Within the close limits of: .0115 and .0145 inch. The earlier practice has been to adjust these spark gaps by bending ,the prongs inwardly or outwardly, as the separate case required, after the plug had been completely assembled, but this is very difficult and time-consuming.

The present invention provides a method of gapping multi-prong spark plugs, particularly spark plugs for use in airplane engines, which ob.- viates the adjustment of the spark gaps by the bending method and establishes a spark gap of predetermined dimension as a part of one of the last steps in the process of manufacture of a spark plug. The method disclosed allows the spark gaps to be maintained within very close limits, so that the lower limit of .0115 inch set forth in the previous paragraph as the lower gap limit may be closely approached and held during the manufacturing process of the spark plug. When the spark gap is maintained close to the lower limit, the spark plug is given a greater gap life because the erosion of electrode material at the gap will not as quickly widen the gap to excessive lengths when a greater amount of material must be eroded away before the gap becomes substantially larger than the upper limit of .0145 inch. This greater gap life will be reflected in better operation of theengine in which the plug is being used and also in obviating the necessity for adjustment after a short period of use of the spark plug.

The present method of gapping spark plugs, by obviating the bending method of adjusting the gap length, allows the juxtaposed faces of the ground electrode vis-a-vis the center electrode to be positioned in substantially parallel relation Ill with the center electrode so that a greater gap area is available as a sparking surface. This gives the advantage of further increasing the amount of material that must be eroded away by sparking before the gap length approaches a prohibitive length.

It is therefore a principal object of this invention to provide a method of gapping spark plugs at the time of assembly which maintains the gap between a multi-prong ground electrode and a cylindrical center electrode substantially uniform with the inner surfaces of the prongs facing the center electrode in substantially parallel relation tion of the related elements of the structure, to-

various details of construction, to combination of parts and to economies of manufacture, will be apparent to those skilled in the art upon consideration of the following description and appended claims, reference being had to the accompanying drawing forming a part of this specification wherein like reference characters designate corresponding parts in the several views. I

Fig. 1 is an elevational view, partly in section, showing a spark plug to which the invention has been applied. I

Fig. 2 is an exploded view, showing the parts of the spark plug just before being assembled.

Fig. 3 is an exploded view of the electrodes of the spark plug.

Fig-4 is an elevational view, in section, showing the electrodes in positionprior to the centering of the ground electrode.

Fig. 5 is a sectional elevation, showing the electrodes in centered relation just after being permanently fixedin position.

Fig. 6 is a plan view, taken along the line 66 ofFlg. 1.. n

- Referring to the drawing, particularly to Fig. 1, a metallic shell member I0 is provided having a lowerexternally-threaded portion ll adapted to .be used to'position the completed spark plug in the block of an engine. The shell I is provided with a serie of counterbores of different internal diameters to form a series of shoulders. One shoulder [2, preferably positioned within the externally-threaded portion l I, is adapted to seat a gasket 13 cooperating between the shoulder I2 and a lower shoulder I4 of an enlarged portion l5 of a ceramic insulator IS. A second shoulder H in the shell I0 is formed adjacent an upper shoulder l8 of the enlarged portion ii of the insulator and is the terminus of a bore I9 which is internally threaded adjacent its upper end to cooperate with a threaded sleeve which extends outwardly from the shell l0 and forms a shielding portion of the spark plug as is well known in the art.

The lower end of the sleeve 20 is a face 20a which cooperates with the upper face of a ring-like compression member 2| embracing the attenuated upper portion of the insulator [6' adjacent the upper shoulder I8. The lower face of the compression member 2| is tapered at Zia so that its outer edge Zlb cooperates with the upper side adjacent the outer edge of'an Inconel spring 22 whose lower inner edge Rois given a curvature to cooperate with the upper shoulder 18 of the insulator I6. The sleeve 20 is provided with a hexagonal portion 23 whereby a tool, such as a wrench, may cooperate therewith to apply a turning torque to the sleeve to advance the sleeve into the bore. is of the shell. ill in order to apply pressure to the spring member 27.- which thrusts the insulator l5 downwardly against the gasket I3 to form a leakproof joint between the shell l0 and the insulator It. It may he prefera ble, after the parts are in completely assembled relation, to braze the shell Ill and the sleeve 20 together by a fillet 24 adjacent the upper end of the shell Ill.

The insulator i6 is provided with a firing tip portion l-Ba from which projects a center electrode 25 positioned in a. longitudinal bore 21- of the insulator, being conveniently sealed in the insulator in a manner well-known in the art. The center electrode 25 projects from the lower end of the shell in. and cooperates with; a multiaprong ground electrode 21 which is. positioned with a shallow counterbore it formed in the shell from its. lower end,

The ground electrode 11, being oil themultiprong type, is preferably provided with three inwardly-extending prongs. 2111, connected by a circumferential web 25 whose outside diameter is slightly smaller than the inside diameter of the counterbore 282 so that the ground electrode. 21 may be moved a suilicient distance with reference to the counterbore 28 to allow centering between the prongs 21a, and the tip; of the, center electrode 25 as will be described hereinafter.

The ground electrode is prefabricated. by a method disclosed in an application of: C;,J'. Dusrseau, Serial No. 476,528. filed February 20?, 19.43, assigned to the same assignee, whereby the. inner sparking surfaces of the prongs. 210. are formed tov fall in the surface: of a cylinder; which been; a predetermined relation tothe cylindrical. surface of the tipoi the center electrode 2!: with which these inner surfaces are adaptedto cooperate to form the spark gap. Theoutside. diameter of the. center wirev 2!. is preferably maintained plus or minus one-thousandth and the diameter of the cylinder, within which the: innensurhce of the prongs 21a. fall, is made larger than the outside diameter 01 the center electrode 25 by twice the lower limit (viz., .0115: inch): or the. spatk gap which is desired between the electrode portions. This diameter of the theoretical cylinder may be formed by a careful dimensioning of the dies which are used in the process of forming the ground electrode 21 or, if preferred, the surfaces may be reamed to desired size. With these predetermined dimensions of the electrodes the process of assembling the spark plug may be carried out as will now be described.

Referring particularly to Fig. 2 which shows an exploded view or the elements 0! the spark pl s. a template 30 is provided whose perimetrical edge 3| is slightly tapered to fit snugly into the counprovided with terbore 28 of the shell Ill. The template III is further provided with a center bore 32 with which a partially-split tubular sleeve 33 is adapted to cooperate. The outer surface of the sleeve 33 adjacent the split portion is given a slight taper so that when it is thrust into cooperative relation with the bore 32 a constricting action takes place, causing the sleeve to act as a. chuck to firml grip the lower end of the electrode 25 when it in pooltioned inside the tubular sleeve 31. I Tubular sleeve 33 may also be made from sheet stock of uniform thickness equal to the length of the desired gap, the stock being rolled to tubular iorm.

with the parts disassembled as shown in Fig. 2, the gasket "is first positioned against the shoulder H on the shell 10, then the insulator, assembled to embrace the electrode "2!, is positioned inside the shell ID with the spring member I! and the compression member 2| cooperating with the upper shoulder I8 01 the insulator with the lower shoulder ll of the insulator contacting the gasket II on the shoulder It 01! the shell asls most clearly shown in Figs. 1 and 3. The sleeve member is then threaded into the threaded bore of the shell I. and screwed into the shell II to loosely maintain the parts 0! the spark plug in operative relation, At this point the template I. is placed in cooperative relation with the counter-bore ll with the firing tip 01 the electrode 2| projecting through the aperture 32 o! the template. The tapered tubular sleeve- Sl is then positioned between the bore 32 and the firing tip of the center electrode 25' and is manually thrust inwardly so as toflrmly grip the two parts tohold thomin centered relation. By this means the firing tip of the center electrode 26 iscentered with relation to the counterbore 28 of the 511511 II and consequently is also centered generally with the shell "1..

When this centering action is completed, the sleeve 20- is advanced into the shell II by its threaded relation with the bore I! of the shell to place a small compression upon the spring member' 22 through the action of the intermediate compression member 11. This place a compresslon' upon the gasket l3 and holds the component parts of the spark plug firmly in operative position. Thereafter the tubular sleeve 3! is re- I tive condition or the spark plug. At this time it is convenient to braae the sleeve 20 and the shell Wtogether by'means of the fillet: 'Il'iisstep of the assembly operationplaces the insulator M,

the shell ID, the sleeve and the spring member 22 in final operative relation which will not be changed throughout the life of the spark plug.

Referring now to Fig. 4, the ground electrode 21 is placed in positionin the counterbore 28 to rest on a shoulder 28a so that the prongs 21a embrace the firing tip of the center electrode 25. The tubular' sleeve 33 is again utilized to center the ground electrode 21, locating in this instance from the firing tip of the center electrode 25 to move the ground electrode 21 in properly centered relation'therewith, The outside diameter of the ground electrode 21 is slightly smaller than the inside diameter of the counterbore 28 so that ample clearance is provided to allow displacement of the ground electrode in the counterbore 28 through the action of the tapered sleeve 33 to allow the ground electrode to be accurately centered about the firing tip of the center electrode 25 by locating from the firing tip. Inasmuch as the cylinder in whose surface the inner faces of the prongs 21a fall is given a predetermined dimension with relation to the outside diameter of the center electrode 25, the tubular sleeve 33, by having its outer surface slightly tapered, will center these two portions to provide a uniform spark gap of the desired dimension between the three prongs 21a and the center electrode. A ring 34 of brazing material, such as silver solder, brass, or the like, which may be conveniently split for easy fabrication, is then dropped into position adjacent the juxtaposed faces of the perimetrical edge of the ground electrode 21 and the inside wall of the counterbore 28. The spark plug is conveniently turned so that the firing tip of the center electrode 25 points upwardly so that the force of gravity may maintain the ring 34 in position. Thereafter heat, preferably by means of induction heating, is applied to the parts to braze them permanently in position as is well known in the art. The ring 34 of brazing material through the action of heat flows between the ground electrode 21 and the counterbore 28 as is clearly shown at 34a in Fig. 5.

After the parts have become cooled to room temperature, the tapered tubular sleeve 33 is removed from its position between the firing tip of the center electrode 25 and the prongs 2111, the space forming the spark gap between the two electrode portions has the predetermined length I as established by the given dimensions closely held in the fabrication of the two electrode portions.

The brazing compound, of which the ring 34 is composed, is selected of material which is not attacked by lead compounds which are generally a component part of high octane fuels which are ordinarily used in aircraft engines. The positioning of the brazing compound between the ground electrode 21 and the shell l0 creates a highly-conductive bond between the two parts which tends to maintain the temperature of the ground electrode 21 as low as possible during use in an airplane engine so that pre-ignition may be prevented.

It is to be understood that the above detailed description of the present invention is intended to disclose an embodiment thereof to those skilled in the art, but that the invention is not to be construed as limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawing, since the invention is capable of being practiced and carried out in various ways without departing from the spirit of the invention. The language used in the specification relating to theoperation'and function of the elements of the inventionis employed for purposes of description and not oflimitation, and it is not intended to limit the scope of the following claims beyond the requirements of the prior art.

What is claimed:

l. The method of gapping a ceramic spark plug having a multi-prong ground electrode separate of the shell with a gap having a predetermined dimension comprising, centering the center electrode with its enclosing ceramic insulator within the shell and fixing the insulator in operative position in the shell, holding the multi-prong ground electrode in position in the shell by locating from the center wire to obtain the predetermined spark gap and welding or brazing. the ground electrode permanently in position in the shell While being held in such position.

2. The method of gapping a spark plug having a; unitary multi-prong ground electrode to form a predetermined spark gap with the center electrode comprising, centering the center electrode with reference to the shell and fixin the insulator in operable position in the shell, positioning the ground electrode loosely with reference to the shell by locating from the firing tip of the center electrode at a point to give the predetermined spark gap between the prongs of the ground electrode and the center electrode, and brazing or welding the ground electrode permanently to the shell while holding the predetermined gap.

3. The method of gapping a ceramic spark plug having a unitary ground electrode with several prongs cooperating with a substantially cylindrical tip of a center wire to form a predetermined spark gap comprising, positioning a center wire with its insulator in center position with reference to the shell and fixing the insulator in operable position in the shell, placing the ground electrode over the center wire tip and holding the ground electrode a predetermined distance from the center wire by a tubular-holding device adapted to fit over the tip of the center wire and between the prongs of the ground electrode and the center wire, said tubular-holding device having a wall thickness equal to the predetermined spark gap to be established, brazing or welding the ground electrode permanently into operable position in the shell and thereafter removing the tubular-holding device from its position between the center wire and the prongs of the ground electrode to establish the predetermined spark ap.

4. The method of gapping a ceramic spark plug having a unitary multi-prong ground electrode cooperating with a center electrode comprising, holding the center electrode with its enclosing ceramic insulator in centered relation with the shell, fastening the insulator in operable position in the shell with the center wire in centered relation with the shell, placing the ground electrode in operative position in the shell and holding th prongs of the ground electrode a predetermined distance from the center electrode by means of a tapered sleeve having a. wall thickness equal to the predetermined spark gap to be established, welding or brazing the ground electrode permanently in position in the shell while being held in operable position, and thereafter removing the tapered sleeve from between the center and ground electrodes to form the predetermined spark gap.

5. The method of gapping spark plugs having a unitary ground electrode with several prongs juxtaposed in gap relation with a center wire comprising, providing a recessed seat in the shell adjacent the firing end of the spark plug to receive the ground electrode, fixing the center wire with its enclosing insulator in operative position in the shell by centering from the walls of the recess, fixing the insulator in operable position in the shell, placing the ground electrode in operative position in the recess, the outside diameter oi the ground electrode being substantially smaller than the inside diameter of the recess to allow limited lateral movement of the ground electrode, holding the ground electrode in the recess with the prongs thereof at predetermined distance from the center wire by a sleeve having a predetermined wall thickness substantially equal to the spark gap to be determined, brazing or welding the ground electrode in position in the recess of the shell while being held in such position and thereafter removing the sleeve from a unitary ground electrode with several prongs to form a predetermined spark gap comprising,

providing a cylindrical center wire of predetermined diametrical dimension and a ground electrode having several prongs whose gap surfaces lie in the surface of a, cylinder whose radius is larger by approximately the predetermined spark gap than the radius of the center wire, fixing the center wire with its enclosing insulator in operative position in the shell by centering from a recess seat in the shell adjacent the firing end of the spark plug, placing the ground electrode in operative position in the recess of the shell, said ground electrode being slightly smaller than the recess to allow limited lateral movement of the ground electrode with reference to the center wire, holding the ground electrode in the recess by a tubular-holding device positioned between the center wire and the prongs of the ground electrode, brazing or welding the ground electrode in position in the recess of the shell while held in centered relation with the center wire, and thereafter removing the sleeve to form the spark gap of predetermined dimension.

ROBERT K. CHRISTIE. 

